Partial predilution dilution chilling

ABSTRACT

A dewaxing process is described wherein a residual waxy petroleum oil stock characterized by having a viscosity greater than about 75 SUS at 210*F. and containing less than about 10 percent of material boiling below about 950*F., is mixed with at least about 0.3 volumes of a dewaxing solvent per volume of residual waxy oil stock, thereby depressing the cloud point of same. In one embodiment of the invention, the resultant mixture is introduced into a cooling zone, at a temperature above the depressed cloud point of the oil. Precooled dewaxing solvent is incrementally added to the cooling zone which is divided into a plurality of stages with agitation means present in each of the stages. The resultant solvent-oil mixture is cooled and agitated as it passes through the cooling zone, thereby reducing the temperature of the oil to below its depressed cloud point and precipitating at least a portion of the wax therefrom. A residual oil stock of diminished wax content is thereafter recovered. In a second embodiment of the invention, the waxy oil stock is introduced into the cooling zone in the absence of solvent at a temperature above the cloud point of the oil. Precooled dewaxing solvent is introduced incrementally into the initial stages of the cooling zone, coming into contact with the waxy oil and depressing its cloud point. The oil is gradually cooled to a temperature no less than the depressed cloud point of the oil whereupon additional precooled dewaxing solvent is added to the oil in the remaining stages of the cooling zone, thereby gradually cooling the oil to a temperature below the depressed cloud point and precipitating at least a portion of the wax therefrom.

[451 Nov, 26, 1974 PARTIAL PREDILUTION DILUTION CHILLING t [75]Inventors: David A. Gudelis; David H. Shaw,

both of Sarnia, Ontario, Canada [73] Assignee: Exxon Research andEngineering Company, Linden, NJ.

[22] Filed: Aug. 29, 1972 [21] Appl. No.: 284,647

Primary Examiner-Herbert Levine [57] ABSTRACT A dewaxing process isdescribed wherein a residual waxy petroleum oil stock characterized byhaving a viscosity greater than about 75 SUS at 210F. and containingless than about 10 percent of material boiling below about 950F., ismixed with at least about 0.3 volumes of a dewaxing solvent per volumeof residual waxy oil stock, thereby depressing the cloud point of same.

In one embodiment oi the invention, the resultant mixture is introducedinto a cooling zone. at a temperature above the depressed cloud point ofthe oil. Precooled dewaxing solvent is incrementally added to thecooling zone which is divided into a plurality of stages with agitationmeans present in each of the stages. The resultant solvent-oil mixtureis cooled and agitated as it passes through the cooling Zone, therebyreducing the temperature of the oil to below its depressed cloud pointand precipitating at least a portion of the wax therefrom. A residualoil stock of diminished wax content is thereafter recovered.

in a second embodiment of the invention, the waxy oil stock isintroduced into the cooling zone in the absence of solvent at atemperature above the cloud point of the oil. Precooled dewaxing solventis introduced incrementally into the initial stages of the cooling zone,coming into contact with the waxy oil and depressing its cloud point.The oil is gradually cooled to a temperature no less than the depressedcloud point of the oil whereupon additional precooled dewaxing solventis added to the oil in the remaining stages of the cooling zone, therebygradually cooling the oil to a temperature below "the depressed cloudpoint and precipitating at least a portion of the wax therefrom.

l .29 4(u) I 8 7 SEPARATOR\ SEPARATOR SEPARATOR I9 Pmmm vz 131850.140

SHEEI 1 BF 2 FIGURE COOLING TOWER PATENTEDNBIZBISH ,740

SHEET 2 BF 2 Figure 2 MEK/TOLUENE (55/45) DILUTION CHILLING ARAMCO 2500BRIGHT STOCK FEED FILTER RATE VERSUS PREDILUTION 4.0 l I I 3.0 T I v u;20?- c: O E

3 LU Ll.

o I I l PREDILUTION SOLVENT/ FEED (V/V) 1 PARTIAL PREDILUTION DILUTIONCHILLING BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention'relates to a process for the dewaxing of a waxy residualpetroleum oil stock. More particularly, this invention relates to asolvent predilution dewaxing process wherein a residual waxy petroleumoil stock is admixed with a solvent prior to the cooling of the oil to atemperature below its depressed cloud point.

2. Description of the Prior Art It is known in the prior art to dewaxpetroleum oil stocks by cooling an oil-solvent solution in scrapedsurface exchangers. In this process the oil and selective solvent areadmixed at a temperature sufficient to effect complete solution of theoil and its contained wax in the solvent. The extent of dilution isdependent upon the particular oil and solvent employed and is adjustedto facilitate easy handling and optimum filtration rates and oil yields.The solution is cooled at a uniformly slow rate, e.g., 1 to 5F. perminute, under controlled conditions so as to avoid any substantialagitation of the solution during precipitation of the wax.Notwithstanding the carefullycontrolled conditions used in this process, there are several deficiencies which hamper successful commercialoperation. Most significant among these deficiencies is the loss of goodheat transfer due to wax deposition on the exchange surfaces. Suchfailing has been repeatedly noted after short periods of operation,e.g., 24 to 48 hours. Associated directly with the loss of good heattransfer is the loss of careful control of the cooling rate and thecorresponding loss of uniform crystal growth. This non-uniform crystalgrowth results in lower filtration rates. The highpressure drop throughthe chilling section also reduces the maximum feed rate obtainable.Physical mashing of the wax crystals by the action of the scrapersmayalso contribute to poor filtration.

It is also known in the prior art to dewax petroleum oil stocks bycooling same in scraped surface exchangers using an incremental'solventaddition technique. In this process, the solvent is added at severalpoints along the chilling apparatus. The waxy oil is chilled withoutsolvent until some wax crystallization occurs and the mixture thickensconsiderably. The first increment of solvent is introduced at this pointand cooling continues. Each incremental portion of solvent is added, ifnecessary, to maintain fluidity until the desired separation temperatureis reached, at which point the remainder of the solvent desired forfiltration is added.

Using this common industrial technique, it is well known, and has beenrepeatedly demonstrated, that the temperature of the solvent should bethe same as that of the main stream at the point of addition. Having thesolvent at a lower temperature causes shock chilling of the slurry atthat point, with resulting formation of crystal fines, and impairment offilter rate; having the solvent warmer throws an unnecessary additionalload on the scraped surface chillers. The bulk of the chilling of theslurry in this well-known process is accomplished through the walls ofthe scraped surface chillers rather than by means of cold solvents.

It is also known in the art, as described in US. Pat. No. 2,361,503 toSchutte et al, to subject lubricating oil fractions to agitation in amulti-staged tower with water or brine, which serves as the coolingmedium. This process suffers from the disadvantage that the coolingmedium is completely immiscible with the wax and oil, and rapidseparation occurs between the feed and water unless the mixture ismaintained in an agitated state or is emulsified. The use of water as acooling medium also practically limits the process to an upfiowoperation. In US. Pat. No. 2,410,483 to Dons et al a twostage dewaxingprocess is described wherein a waxcontaining oil stock, heated above itscloud point, is introduced into an elongated cooling vessel divided intoa plurality of stages. Cold dewaxing solvent is injected into each ofthe stages thereby cooling the oil. The resultant oil solvent/mixture iswithdrawn from the cooling vessel at the temperature of the cloud pointof the oil or no more than about lOF., above the cloud point. Thismixture is then introduced into a pipe wherein additional cold dewaxingsolvent is added thereto in the substantial absence of any agitation,thereby cooling the oil to below its cloud point and precipitating waxtherefrom. The process contemplates no substantial agitation in thecrystallization phase of the dewaxing process. As a consequence, thefiltration rates are inferior to other dewaxing processes.

In US Ser. No. 129,973, filed Mar. 31, 1971 now US. Pat. No. 3,773,650,which application is a C.l.P. of US. Ser. No. 17,869, filed Mar. 9, 1970(now abandoned) which is, in turn, a C.I.lP. of US. Ser. No. 666,268,filed Sept. 8, 1967 (now abandoned), the disclosures of which areincorporated herein by reference, there is taught a method for dewaxingoils, i.e., dilution chilling, wherein a waxy oil stock is introducedinto a cooling zone divided into a plurality of stages. Dewaxing solventis introduced into the cooling zone at a plurality of spaced pointssituated along the cooling zone, coming into contact with the oil. Highlevels of agitation are provided in at least a portion of thesolvent-containing stages, thereby providing substantially instantaneousmixing of the solvent and oil, e.g., within a second or less. As the oilpasses through the cooling zone, it is cooled to a temperaturesufficient to precipitate at least a portion of the wax therefromresulting in the formation of a wax slurry wherein the wax particleshave a unique crystal structure, thereby providing superior filteringcharacteristics such as high filter rates and high dewaxed oil yields.While the process of Ser. No. 129,973 overcomes many of thedisadvantages of the prior art, it has not been found to be veryeffective with waxy residual lubricating oil stocks, such as residualbright stocks. The filter rates with these heavy oils have been found tobe quite low and there is considerable incentive to improving thefilterability characteristics'of these oils.

SUMMARY OF THE INVENTION In accordance with the invention, it has nowbeen discovered that waxy residual lubricating oil feedstocks can bedewaxed and that substantial improvements in filtration rates can beobtained by use of the solvent predilution process of the subjectinvention.

The process is particularly suitable for dilution chilling dewaxing andcomprises, in one embodiment of the invention, prediluting a waxyresidual oil with at least about 0.3 volumes ofa predilution solvent pervolume of residual oil stock, resulting in the depression of the cloudpoint of the oil stock. The process feedstock comprises a residual waxypetroleum oil stock characterized by having a viscosity greater thanabout SUS at 210F. and containing less than about percent of materialboiling below about 950F., (all temperatures are reported at atmosphericpressure, unless otherwise stated).

The cloud point of the oil is defined as the temperature at which acloud or haze of wax crystals first appears when an oil is cooled underprescribed conditions (modified ASTM D2500-66 procedure). Predilution,"as the term is used herein, refers to the mixing of solvent and oilprior to cooling of the oil to a temperature below its depressed cloudpoint.

The resultant solvent-oil mixture is introduced into a cooling zonedivided into a plurality of stages, at a temperature above the depressedcloud point of the oil. Additional dewaxing solvent, which may be thesame or different than the predilution solvent used to form the initialsolvent-oil mixture, is introduced into at least a portion of the stagesand high levels of agitation are maintained in at least a portion of thesolventcontaining stages thereby providing efficient mixing of solventand oil. The high levels of agitation referred to above are onlynecessary during the initial phases of wax crystal nucleation andgrowth. Once good crystal growth is effected, lower agitation levels maybe used, e.g., in the later stages of the cooling zone.

The solvent-oil mixture is cooled as it passes through the cooling zoneto a temperature below the depressed cloud point of the waxy oil stock,thereby precipitating at least a portion of the wax therefrom, and aresidual oil stock of diminished wax content is recovered.

In another embodiment of the invention, the predilution of the oil isconducted in situ, i.e., within the cooling zone itself. To this end,the feedstock is introduced into the cooling zone at a temperature aboveits cloud point and in the substantial absence of solvent. At leastabout 0.3 volumes of solvent per volume of oil is added to the initialstages of the cooling zone, coming into contact with the oil stock andforming an oil-solvent mixture. The mixture is gradually cooled, as itpasses through the initial cooling stages, to a temperature no less thanthe depressed cloud point of the oil stock. Thereafter, additionalsolvent is introduced into at least a portion of the remaining stages ofthe cooling zone, and the oil is further cooled to a temperature belowits depressed cloud point thereby precipitating at least a portion ofthe wax.

Although it is preferred that a substantial portion of the cooling ofthe oil be provided by the contacting of same with prechilled solvent,it is contemplated that other cooling means, such as autorefrigeration,wherein cooling is effected in part by vaporization of solvent, may alsobe employed.

The feedstock that is used in the process of the invention comprises aresidual waxy oil stock having an initial boiling point above about800F., with less than about 10 percent (by weight) of material boilingbelow about 950F. and less than about 50 percent (by weight) of materialboiling below about 1,050F. The oil is further characterized by having aviscosity greater than about 75 SUS at 2 lOF. and ranging between about75 and 300 SUS, preferably between about 100 and 200 SUS and mostpreferably between about 125 and 175 SUS at 210F.

The residual oil contains the most difficulty vaporizable components ofpetroleum hydrocarbons including asphaltenes and pitch, which areundesirable not only in the finished lubricating oil product, but alsoin the LII intermediate refining operations, as discussed in more detailinfra. It is thus preferred, prior to the dewaxing operation of thesubject invention, to remove as much of these components from theresidual oil as possible, such as by a deasphalting operation, e.g.,propane deasphalting. Further, the residual oil may contain aromatic andpolar molecules which would impart undesirable properties to thefinished lube oil product. These molecules may be removed by using suchprocess techniques as solvent extraction, comparatively severe hydrogentreatment and the like either before or after the dewaxing step.

Preferably, the residual oil is derived from a raw lube oil stock, themajor portion of which boils above about 650F. This oil stock can bevacuum distilled with the resultant overhead and sidestreams beingtermed distillates and the bottoms being termed residua or residual oilstocks. Considerable overlap may be encountered in the boiling ranges ofthe distillates and residua dependent, in part, on the efficiency of thedistillation, with certain of the higher boiling distillates containingalmost the same distribution of molecular species as the residua andtherefore showing similar responses to the dewaxing operating variablesof the subject invention. These distillates are therefore included inthe term residual as used herein. The crude sources from which theinstant feedstocks may be obtained are exemplified by the paraffiniccrudes such as Aramco, Kuwait, the Panhandle, North Louisiana, Tia Juanaand the like.

In general, the wax content of the feedstock as defined by the amount ofmaterial to be removed to produce an oil with a pour point in the rangeof +25 to 0F. will vary between about 5 and 35 wt. percent based ontotal feed, preferably between about 10 and 30 wt. percent. The initialpour and cloud points of the oil will range, respectively, between aboutand 175F. and about and 180F.

The predilution solvent is selected from any of the dewaxing solventsknown in the prior art such as the aliphatic ketones having from threeto six carbon atoms, e.g., acetone, methylethyl ketone (MEK),methylisobutyl ketone (MlBK) and the like, the lower molecular weighthydrocarbons such as ethane, propane, butane and propylene, as well asmixtures of the foregoing ketones and mixtures of the ketones withhydrocarbon compounds such as propylene, and aromatics such as benzeneand toluene. In addition, halogenated low molecular weight hydrocarbonssuch as the C -C chlorinated hydrocarbons, e.g., dichloromethane,dichloroethane and mixtures thereof, may be used. Specific examples ofeffective predilution solvents include toluene, MlBK, MEK/toluene,MEK/MIBK and the like.

The depressed cloud point of the oil is dependent, in part, upon thedegree of predilution of the oil with solvent and will preferably rangebetween about 50 and l75F., most preferably between about 50 and F. Ingeneral, the amount of predilution solvent added to the oil will bedependent, in part, on the nature of the feedstock, the cooling zone,the extent of cooling within the cooling zone, i.e., approach to thefiltration temperature, and the desired final ratio of solvent to oil inthe wax/oil/solvent slurry withdrawn from the cooling zone. Preferredamounts of predilution dewaxing solvent range between about 0.3 and 2.0volumes per volume of residual oil stock, most preferred between about0.5 to 1.5 volumes of solvent per volume of oil stock.

The dewaxing solvent that is used during the phase of the dewaxingoperation conducted at a temperature below the depressed cloud point ofthe oil may be the same as or different than the predilution solvent andis selected from the same group of solvents mentioned in connection withthe predilution solvents. Specific examples of suitable dewaxing solventmixtures include methylethyl ketone/methylisobutyl ketone, methylethylketone/toluene and propylene/acetone. The preferred solvents are the C-C ketones with methylethyl ketone being particularly preferred. It isnoted that when the dewaxing solvent is MEK, a particularly preferredpredilution solvent comprises toluene or MIBK.

While all the cooling of the oil stock to the subsequent filtrationtemperature may take place in the dilution chilling zone, this is notnecessarily required for the successful operation of the subjectprocess. In fact only a portion of the cooling need be done therein.Further cooling of the wax/oil/solvent slurry withdrawn from the coolingzone to the filtration temperature may take place in conventionalcooling apparatus such as scraped-surface equipment, anautorefrigeration vessel and the like. A description of this aspect ofthe process is found in U.S. Ser. No. 257,435, filed May 26, 1972, thedisclosures of which are incorporated herein by reference.

Quite surprisingly, the predilution process has been found to bespecific to waxy residual oils as described hereinabove, and is, infact, detrimental to the dewaxing of light distillates. The term lightdistillates" as used herein refers to a feedstock having a 90 percent orend boiling point as high as about 1,050F. and containing at least 50percent (by weight) of material boiling below about 1,050F. In addition,these light distillates usually contain more than about percent (byweight) of material boiling below about 950F. but contain less thanabout 5 percent (by weight) of material boiling below about 650F. Thedistillate is further 1 characterized by having a viscosity at 210Fbelow about 75 SUS.

While the exact mechanism of the predilution process is not known, it isspeculated that trace amounts of asphaltene and pitch components presentin the residual feedstocks, possibly as a result of contamination in thevacuum pipestill or inadequate deasphalting, interfere with theformation of wax crystals of the desired structure. Specifically, it isthought that at some temperature above the depressed cloud point of thefeedstock, these asphaltene and pitch components precipitate from theoil as very small crystals which interfere with the uniform nucleationand growth of the wax crystals. It is contemplated that solventpredilution facilitates solution of these very small crystals, anddelays their precipitation until after the cloud point temperature ofthe oil is reached, at which time, they cocrystallize with the waxcomponents of the oil, thereby substantially reducing wax crystal growthinterference.

It is also thought that predilution techniques in dilution chillingdewaxing reduce the overall viscosity of the oil stock in the criticalearly stages of crystal nucleation and growth thereby removing diffusionlimitations to crystal growth and facilitating the development of largerparticles.

This may be particularly important with the residual fecdstocks and thelike, since the wax crystallizing from such high boiling, high molecularweight feedstocks comprises highly branched paraffins and naphthenes,which have very low crystal growth rates. In contrast, the waxcrystallizing from lower boiling distillate feedstocks, generallycontains predominantly normal paraffins, which have relatively highcrystal growth rates and would therefore not be as sensitive todiffusion limitations.

Further improvements in filter rate may be obtained when dewaxing aidsare used in conjunction with solvent predilution for dewaxing residualfeedstocks. A preferred dewaxing aid comprises a Ziegler type mixednormal alpha olefin copolymer described in more detail in U.S. Ser. No.164,892, filed July 21, 1971, having a number average molecular weightbetween about 2,000 and 60,000 or higher, and having pendant side chainsof C and higher. A particularly preferred dewaxing aid compositioncomprises 38 wt. percent nhexene-l, 26percent n-hexadecene-l, 21 percentnoctadecene-l and 15 percent n-eicosene-l. Other dewaxing aids may alsobe used such as polymeric higher alkyl methacrylates, long-chain alkyl1,2-oxiranes, po-

lymerized higher fatty acid esters of vinyl alcohol, a mixture of atleast two homopolymers of a C C alpha olefin, a Friedel-Craftscondensation product of a halogenated hydrocarbon such as chlorinatedparaffin wax with an aromatic hydrocarbon such as naphthalene, mixturesthereof and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified flow scheme ofa preferred embodiment of the dewaxing process of the subject invention.

FIG. 2 is a graph relating filter :rate to the amount of predilution inMEK/toluene dilution chilling dewaxing of an Aramco 2,500 bright stock.

DETAILED DESCRIPTION Referring to FIG. 1, a waxy lubricating oil stockis taken from tankage and introduced into predilution mixing zone 1 vialine 28 while dewaxing solvent is introduced therein via 1ine-29QAfter asufficient contact time, the resultant solvent-oil mixture is introducedvia line 2 into cooling zone 3, at a temperature above the depressedcloud point of the feedstock. Although not shown, heating means may beprovided in mixing zone 1 to ensure that the feed temperature is abovethe depr'essed cloud point of the oil prior to introduction into thecooling zone. The cooling zone is depicted herein as a vertical coolingtower; however, it is noted that the design is not limited to thisconfiguration. The solventoil mixture enters the cooling tower and intothe first stage of the cooler, i.e., 4(a). Dewaxing solvent is passedfrom storage tank 5 through line 6, and heat exchangers 7 and 8, wherethe solvent temperature is reduced to that sufficient to cool the oil tothe desired temperature. Coolant enters the heat exchangers 7 and 8through lines 24 and 25, respectively and leaves through lines 26 and27. It will be apparent to those skilled in the art that the exactsolvent temperature employed will depend upon the amount of oil to becooled and the amount of solvent to be added to the oil, i.e., thedegree of oil dilution which is sought during the filtration step.

The solvent leaves the heat exchanger 8, through line 9, and entersmanifold 10. The manifold comprises a series of spaced solvent inletpoints 11 to the several stages of the cooling tower 3. The rate ofsolvent flow through each inlet is regulated by flow control means (notshown) and is adjusted, so as to maintain a desired temperature gradientalong the height of the cooling tower. Preferably, the incrementalsolvent addition is such that the chilling rate of the oil is belowabout lF./minute and most preferably between about 1 and 5F./minute. Ingeneral, the amount of solvent added thereto will be sufficient toprovide a liquid/solid weight ratio between about 5/1 and 100/1 at thedewaxing temperature and a solvent/oil volume ratio between about 1.0/1and 7/1.

The-first portion or increment of solvent enters the first stage, 4a, ofthe cooling tower 3, where it is substantially instantaneously mixedwith oil due to the action of the agitator 12a. The agitator is drivenby a variable speed motor 13 and the degree of agitation, as defined inmore detail below, is controlled by variation of the motor speed, withdue allowance for the flow rate through the cooling tower. It is notedthat while a rotating blade is shown as the agitation source, any othermixing means that is able to produce the high levels of agitationrequired can be used herein. The oil-solvent mixture may pass upwardlyor downwardly through the cooling tower 3 (downwardly flow only has beenshown). At various heights along the cooling tower, additionalprechilled solvent is introduced to each of the several stages 4,through inlets 1 1 so as to maintain substantially the same temperaturedrop from one mixing stage to the next and at the same time to providethe desired degree of dilution. It should be noted that any number ofstages up to 50 may be employed; however,

at least six is preferred.

The cooling of the oil stock continues to a temperature substantiallybelow the depressed cloud point of the oil stock, thereby precipitatingat least a portion of the wax therefrom and forming a wax-oil-solventmixture.

The oil-solvent solution with precipitated wax passes from the finalstage of the cooling tower through line 14 to wax separation means 15.If desired the wax-oil solvent mixture may be further cooled byconventional means not shown. Any suitable separation means such asfiltration or centrifugation may be employed. The wax-solvent mixture isremoved through line 16 and the solvent recovered therefrom in asuitable separating system 19, which preferably comprises stripping withan inert gas such as nitrogen, steam or air, or straight distillation.The solvent leaves the separator 19 through line 17 and the wax exitsthrough line 18.

The oil-solvent mixture leaves separator through line 20 and passes tooil separation means 21. Any suitable means to effect this separationmay be used, such as distillation, selective adsorption, or strippingwith an inert gas such as nitrogen, air or steam. The solventfree oil isremoved from the separator and recovered through line 22. The solvent isremoved through line 23 and may be recycled directly to the dilutionchilling tower or first scrubbed to remove impurities before reuse.

As indicated previously, the degree of agitation, during the initialstages of crystal nucleation and growth, must be sufficient to providesubstantially instantaneous mixing of solvent and oil, i.e., preferablywithin a second or less, The degree of agitation required in the processcan be achieved by increasing the agitator rpm, when all other mixingvariables, e.g., flow rates through the mixer, vessel and agitatordesign, viscosity of the ingredients and the like, are maintainedconstant, so that the modified Reynolds Number (Perry, ChemicalEngineers Handbook, 3rd, pp. 1224, McGraw-Hill, New York, 1959), Me,which is defined by the equatron:

where L agitator diameter, ft.

1 liquid density, pound/feet n agitator speed, revolution/second uliquid viscosity, pound/feet second ranges between about 200 and about150,000. The dimensionless ratio of cooling tower diameter to agitatordiameter is between about 15/1 and about 10/1 and the ratio of theimpeller blade length to impeller blade width ranges from about 0.75 to2 and preferably from about 1 to 1.5. The ratio of the mixing stageheight to the diameter of the stage will generally range from about0.2/1 to about l/l. A turbine type agitator is preferred, however, othertypes of agitators such as propellers may be used.

The cooling tower may or may not be baffled, but a baffled tower ispreferred. Each stage will generally contain from about two to eightbaffles and preferably from two to four baffles, located about the outerperiphery of each stage, The width of the baffles may range from about 5to 15 percent of the diameter of the tower. In general, the dimentionalratio of the crosssection of the restricted flow opening to thecrosssection of the tower will be between about H20 and about l/200.

The cooling tower of the present invention is preferably operated at apressure sufficient to prevent flashing of the solvent. Atmosphericpressure is sufficient when the ketones are employed as solvent;however, superatmospheric pressures are required when low molecularweight hydrocarbons such as propyleneacetone and relatedautorefrigerative solvents are used. As noted above, however, insituations where propylene-acetone and related autorefrigerative typesolvents are used, low pressures will be required. A process combiningboth vaporization of the solvent to provide in situ refrigeration anddirect cooling from cold dewaxing solvent is disclosed in US. Pat. No.3,658,688 patented Apr. 25, 1972, the disclosures of which areincorporated herein by reference.

The recovered lube oil products may, if so desired, be subjected tovarious finishing operations such as clay contacting, hydrofinishing,acid treatment and the like.

PREFERRED EMBODIMENT The invention will be more apparent from theworking examples set forth hereinbelow.

EXAMPLE 1 A laboratory experiment was performed in a 6 inches diametersingle stage batch unit provided with a 2 inches diameter flat-bladedturbine impeller, a means for solvent introduction and an overflowdevice to maintain a constant volume of slurry. This batch unit does notcompletely duplicate continuous multi-staged operations but has beenfound to give approximately EXA P equivalent results. M LE 2 Thefeedstock used in this example was a deasphalted Thefexpehmhhtsdlsclosed 1h E pl 1 Supra; were phenol-extracted residual distillationfraction from an rerun m a wmmuous 16 Stage pll'ot compnsed of Arabianlight crude. having less than 10 percent of maa 6 inches dhhheter towerq pp with 2 inches terial boiling below 975F. and less than 50 percentof "lmeteh 6 bladeflat la disc turbine 'fiE l l H material boiling below1,150F. The feedstock had an Experiments were also conducted in whichthe P initial pour point f 145%? an i i i Cloud point f lution solventwas toluene and the composition of the 150F., a viscosity at 210F. of140 sus, and required MEK/wluene dewaxing Solvent was adjusted to giveremoval f 5 percent (wt) dry wax to give a bright the desired endsolvent composition of MEK/toluene,

stock lubricating oil product with a +F. pour point. 55/45 LV%i at thedesired final Solvent/feed ratio This feed is hereinafter referred to asan Aramco 2500 of I H bright stock. The data displayed below in Table 11relate degree of Methylethyl ketone/toluene, 55/45 LV%, was used aspredilution, cloud point reduction in the oil stock and both thepredilution solvent and as the dewaxing soll5 process performance asmeasured by filtration rate. vent during the chilling operation. Thesolvent compo- The data are displayed for the lab single stageequipsition in the dilution chilling dewaxing operation was ment inaddition to the pilot plant 16 stage dilution adjusted to obtainapproximately a 4:1 final solvent/oil chilling tower.

TABLE 11 Feed Filter Rate USG/fP-hr v Cloud Point F. Pilot Unit LabSingle Stage Predilution MEK/Tol MEKfI'oI MEK/Tol V/V Tolu- (55/45)Toluene* (55/45) Toluene* (SS/45) ene Composition of predilutionsolvent. Solvent composition to tower adjusted to give outlet MEX/T01(55/45).

dilution ratio. 6th; variables such as average chilling The dataindicate that predilution is an effective rate, agitation levels and thelike are displayed below means for increasing the overall filtrationrate in the in Table 1. Excess slurry comprising precipitated wax,dewaxing of waxy residual feedstocks. Additionally, the oil and solventwas allowed to overflow the apparatus. 5 data indicate that predilutionsolvent systems such as wh the slurry h d a ifi d t pe t th mixtures ofmethylethyl ketone and toluene perform as contents were drawn off andchilled further by conven- Well, if hot better than P Solvent SystemsSuch 33101- tional means in order to reach a common filtration tem-Ilene in rrying Out h process Objectives. The advanperature. I tage ofusing toluene rather than MEK/toluene (55/45 p A LV%) as the predilutionsolvent relates to the greater The data Show that with pre ilution n trange of cloud point depression obtained with toluene for a 0.5 to 1.5volumes of solvent/volumes of oil, DWO filgiven ratio of predilutionsolvent/feed. Since the prediter rates were increased by nearly 100percent. The lution solvent, in addition to the feed, has to be chilleddata, which have been graphed and are displayed in from a few degreesabove the dpress e d Eloud poirTtto FIG. 2, show the tremendousenhancement in filtration the filter temperature, there are obvioussavings in rerate when predilution is used. It is further noted thatfrigeration from operating with the lowest possible dethe majorimprovement is observed in the filtration rate pressed l d im as pp 10the dewaxed 0h Yields which remain The use of a single solventcomposition for predilufairly constant throughout the Various ruhstionand dilution chilling has the obvious advantage 5 TABLE I MEK/TOLUENE.(/45 LV%) DILUTION CHILLING DEWAXING QF ARAMCO 2500 BRIGHT STOCKLaboratory Simulation of a 16 Stage DiIutiOn Chilling Tower Stage volumeI500 ccs Impeller centrally mounted 2" diameter 6 bladed, flat bladed.disc turbine Agitation 770 rpm Solvent for predilution, stage injectionand filter wash: MEK/toluene 55/45 LV% Solvent injected into stage at20F.

Filter temperature. +5F. Wash k Filter Time.

Dilution Chilling Performance Cloud Dilution Wash DWO Filter DWO FeedDWO Run Predilntion Point Start End End Feed Rate Yield Filter Rate PourPoint No. V/V F. F. F. V/V V/V USGIft -hr. wt.% on feed uso/re-m.

the Aramco 2,500 bright stock waxy raffinate, described in Example 1.was introduced into the chilling zone in the absence of solvent.Dilution chilling was performed with F. MEK/toluene (55/45 LV'/() andthe effect of varying the feed temperature on effective predilution andperformance is shown in Table IV below.

TABLE IV EFFECT OF IN SITU PREDILUTION ON MEK/TOLUENE DILUTION CHILLINGDEWAXING ARAMCO 2500 BRIGHT STOCK Initiation of Wax Crystallization Instage Solvent/Feed* Stage Feed Filter Rate Feed Temperature, F. numberV/V in Stage Temperature, F. USGIft -hr.

Effective predilution EXAMPLE 3 TABLE III The data confirm that in situpredilution is an alternative means ofincreasing the overall filtrationrate in the dewaxing of residual feedstocks, althrough it suffers fromthe disadvantage that less effective use is being made of the first fewstages prior to wax crystallization.

EXAMPLE 5 This example illustrates the detrimental effect of predilutionon a phenol extracted light distillate feedstock from a Western CanadianCrude. The feedstock KETONE DILUTION CHILLING DEWAXING ARAMCO 2500BRIGHT OCK EFFECT OF PREDILUTION ON DWO FILTER RATE Lab single stagesimulation of 16 stage dilution chilling. Same solvent composition usedfor predilution and subcloud point cooling. Dilchill solvent temperature20F. Agitation 770 rpm (2" impeller) chilling rate 2F/minuteSolvent/feed to filter 4/1.

'DWO Filter Rate, wash filter time, relative to MEK/toluene case with nopredilution The data indicate that a similar beneficial effect ofcontained less than 5 percent of material boiling below predilution onthe filter rate was obtained using all 660F. or above 890F., and itsviscosity at 210F. was

three solvents.

EXAMPLE 4 This example demonstrates the performance advantage obtainedby in situ predilution. The experiments were carried out in thelaboratory single stage unit, and

41 SUS. The initial feed pour and cloud points were F. and F.respectively, and it required the removal of 22 percent dry wax to yielda lubricating oil with a 0F. pour point. The process conditions aredisclosed in Table V. The data typify the effect of predilution on lightdistillates.

TABLE v MEK/MIBK CHlLLlNG ON A LOW BOILING DlSTlLLATE FROM A WESTERNCANADIAN CRUDE Effect of Predilution Ahead of Tower Lab single stagesimulation of 16 stage dilution chilling.

l 130 rpm. Chilling rate 2F./min. Filtered at F.

Solvent/Feed (wt/wt) Performance In Feed DWO Filter DWO Yield Run totower to as Wash Rate Wt.% on No. (predilution) filter to filter USG/fthr. feed EXAMPLE 6 b. introducing said first mixture at a temperature lnthis example, the feedstock was a phenol-extracted distillate from aWestern Canadian crude, with 45 percent of material boiling below 950F.,85 percent of material boiling below l,050F., and also characterized byhaving a viscosity of 63.1 SUS at 210F. and requiring removal of 19percent dry wax to yield a lubricating oil of +20F. pour point. Theinitial feed pour point was 125F. and the initial feed cloud point was130F. In one instance, the feed was introduced into the 16 stagedilution chilling pilot unit, described in Example 2, at l35F., while inanother instance the feed was introduced into the 16 stage pilot unit at155F. under in situ predilution conditions. Other conditions, and thedeleterious effect on performance of in situ predilution obtained byelevating the feed temperature is illustrated in Table VI below.

TABLE V] DlLUTlON CHlLLlNG DEWAXING A WESTERN CANADIAN MEDIUM BOlLlNGDlSTlLLATE EFFECT OF IN SITU' PREDlLUTlON 16 Stage pilot unit. 2"impellers. Solvent MEK/MIBK 45/55 Lvvr. Dilchill solvent at -20F.Agitation 1 I rpm. Chilling rate 2F./min. Filter at +20F.

Feed temperature. F. initial wax cloud point reached at: stage number I4 stage temperature. F. I28 I26 sol\'ent/feed (effective predilution).06 .27

Solvent/Feed to filter 2.8 2.7

-to wash* |.l 0.8 DWO Filter Rate. UsG/ft -hrfl 4.8 4.2

DWO Yield, Wt.% on feed 77.2 67.8

' Wash time filter time above the depressed cloud point of said residualoil stock into a cooling zone divided into a plurality of stages andpassing said mixture from stage to stage of said cooling zone;

c. introducing dewaxing solvent into at least a portion of said stagesof said cooling zone at a plurality of spaced points therealong;

d. mixing said dewaxing solvent: with at least a portion of said firstmixture as it passes from stage to stage of said cooling zone underconditions of high agitation, thereby forming a second mixturecomprising said dewaxing solvent, said predilution solvent and saidresidual oil stock; and,

e. cooling said residual oil stock contained in said second mixture asit passes from stage to stage of said cooling zone, thereby reducing thetemperature of said residual oil stock to below its depressed cloudpoint and precipitating atleast a portion of said wax therefrom undersaid conditions of high agitation.

2. The process of claim 1 wherein said waxy residual petroleum oil stockis characterized by containing less than about 10 percent (weight) ofmaterial boiling below about 950F., at atmospheric pressure, and lessthan about 50 percent (weight) of material boiling below about l,050F.,at atmospheric pressure.

3. The process of claim 1 wherein the solvent in step (a) is a dewaxingsolvent and is selected from the group consisting of aliphatic ketonescontaining from 3 to 6 carbon atoms per molecule, the lower molecularweight hydrocarbons, aromatic compounds, halogenated lower molecularweight hydrocarbons and mixtures thereof.

4. The process of claim 3 wherein said solvent is selected from thegroup consisting of methylethyl ketone, methylisobutyl ketone, tolueneand mixtures thereof.

5. The process of claim 1 wherein the solvent used in step (a) is adewaxing solvent and is the same as or different than the dewaxingsolvent used in step (c).

6. The process of claim 1 wherein the degree of agitation is sufficientto provide substantially instantaneous mixing of solvent and oil.

7. The process of claim 1 wherein the dewaxing solvent used in step (c)is prechilled prior to introduction into said cooling zone.

1. A PROCESS FOR THE DEWAXING OF A WAXY RESIDUAL PETROLEUM OIL STOCKHAVING A VISCOSITY RANGING BETWEEN ABOUT 125 AND 175 SUS AT 210*F, SAIDPROCESS COMPRISING: A. MIXING SAID RESIDUAL OIL STOCK WITH BETWEEN ABOUT0.3 AND 2 VOLUMES OF A PREDILUTION SOLVENT PER VOLUME OF OIL STOCK,THEREBY DEPRESSING THE CLOUD POINT OF SAID RESIDUAL OIL STOCK ANDFORMING A FIRST MIXTURE COMPRISING SAID PREDILUTION SOLVENT AND SAIDRESIDUAL OIL STOCK; B. INTRODUCING SAID FIRST MIXTURE AT A TEMPERATUREABOVE THE DEPRESSED CLOUD POINT OF SAID RESIDUAL OIL STOCK INTO ACOOLING ZONE DIVIDED INTO A PLURALITY OF STAGES AND PASSING SAID MIXTUREFROM STAGE TO STAGE OF SAID COOLING ZONE; C. INTRODUCING DEWAXINGSOLVENT INTO AT LEAST A PORTION OF SAID STAGE OF SAID COOLING ZONE AT APLURALITY OF SPACED POINTS THEREALONG; D. MIXING SAID DEWAXING SOLVENTWITH AT LEAST A PORTION OF SAID FIRST MIXTURE AS IT PASSES FROM STAGE TOSTAGE OF SAID COOLING ZONE UNDER CONDITIONS OF HIGH AGITATION, THEREBYFORMING A SECOND MIXTURE COMPRISING SAID DEWAXING SOLVENT, SAIDPREDILUTION SOLVENT AND SAID RESIDUAL OIL STOCK; AND, E. COOLING SAIDRESIDUAL OIL STOCK CONTAINED IN SAID SECOND MIXTURE AS IT PASSES FROMSTAGE TO STAGE OF SAID COOLING ZONE, THEREBY REDUCING THE TEMPERATURE OFSAID RESIDUAL OIL STOCK TO BELOW ITS DEPRESSED CLOUD POINT ANDPRECIPITATING AT LEAST A PORTION OF SAID WAX THEREFROM UNDER SAIDCONDITIONS OF HIGH AGITATION.
 2. The process of claim 1 wherein saidwaxy residual petroleum oil stock is characterized by containing lessthan about 10 percent (weight) of material boiling below about 950*F.,at atmospheric pressure, and less than about 50 percent (weight) ofmaterial boiling below about 1,050*F., at atmospheric pressure.
 3. Theprocess of claim 1 wherein the solvent in step (a) is a dewaxing solventand is selected from the group consisting of aliphatic ketonescontaining from 3 to 6 carbon atoms per molecule, the lower molecularweight hydrocarbons, aromatic compounds, halogenated lower molecularweight hydrocarbons and mixtures thereof.
 4. The process of claim 3wherein said solvent is selected from the group consisting ofmethylethyl ketone, methylisobutyl ketone, toluene and mixtures thereof.5. The process of claim 1 wherein the solvent used in step (a) is adewaxing solvent and is the same as or different than the dewaxingsolvent used in step (c).
 6. The process of claim 1 wherein the degreeof agitation is sufficient to provide substantially instantaneous mixingof solvent and oil.
 7. The process of claim 1 wherein the dewaxingsolvent used in step (c) is prechilled prior to introduction into saidcooling zone.